Ram air inlets have been used on commercial and military aircraft for a variety of different purposes. For example, ram air inlets have been used to provide air for propulsion and auxiliary power systems, as well as for various types of cooling systems. Ram air inlets have also been used to provide air for passenger cabin air conditioning systems and other on-board environmental control systems (ECSs).
FIG. 1A, for example, is a cross-sectional view of a portion of an aircraft 100 having an ECS ram air inlet 101 configured in accordance with the prior art. The ram air inlet 101 includes a conventional, 2-piece ram air inlet door 110 spaced apart from an inlet lip 108 to form a variable “throat” or inlet opening 124. The 2-piece inlet door 110 includes a forward door 112 and an aft door 114. The aft door 114 is set an angle relative to an inlet sidewall 109 to form a diffuser passage 132 leading to an ECS inlet duct 126. The ECS inlet duct 126 provides air to an ECS (not shown). A first hinge 116 pivotally connects a leading edge of the forward door 112 to an inlet housing 115. A second hinge 117 pivotally connects a leading edge of the aft door 114 to a trailing edge of the forward door 112. Tracks 118 (e.g., slider blocks) slidably support a trailing edge of the aft door 114. A door drive mechanism 122 (e.g., a 4-bar linkage) is operably coupled to the aft door 114 near the second hinge 117.
In operation, extension of the drive mechanism 122 moves the inlet door 110 toward the inlet lip 108, thereby restricting the inlet opening 124 and reducing the flow of ram air to the ECS inlet duct 126. The tracks 118 allow the trailing edge of the aft door 114 to slide forward as the two-piece inlet door 110 folds inwardly about the second hinge 117. Conversely, retraction of the drive mechanism 122 moves the inlet door 110 away from the inlet lip 108, thereby enlarging the inlet opening 124 and increasing the flow of ram air to the ECS inlet duct 126. The tracks 118 allow the trailing edge of the aft door 114 to slide aft as the two-piece inlet door 110 unfolds about the second hinge 117. A sliding seal 120 can be used to at least partially seal the gap between the trailing edge of the aft door 114 and the inlet housing 115 as the trailing edge moves fore and aft in the tracks 118. Although not shown in FIG. 1A, the inlet door 110 can include a number of additional components including, for example, a bellcrank, a torque tube, bearings, actuator mount, stiffener channel, etc.
The 2-piece inlet door illustrated in FIG. 1A is relatively complex. As a result, it can be relatively heavy and costly to manufacture. Furthermore, excessive dynamic loads on the door components, resulting from flow instability at the inlet opening 124, can cause excessive component wear which in turn leads to more frequent inspections and maintenance to prevent component failure. The flow instability relates to the geometry of the diffuser passage 132. For example, at throat openings less than about 35%, the 2-piece inlet door 110 operates in an unstable flow regime of the diffuser known as “transitory stall,” as discussed in more detail below with reference to FIG. 1B.
FIG. 1B is a graph 128 illustrating the flow performance of the 2-piece door 110 of FIG. 1A. The parameter 2α is measured along a vertical axis 130 of the graph 128, and the parameter L/2b is measured along a horizontal axis 131. As shown in FIG. 1C, the variable “α” relates to the angle formed between the aft door 114 and the sidewall 109. The variable “L” relates to the horizontal length of the diffuser passage 132, and the variable “b” relates to one-half the size of the ram air inlet opening 124. A first plot 134 represents a boundary between an attached flow regime (i.e., a stable flow regime) and a transitory stall regime (i.e., an unstable flow regime). A second plot 136 illustrates operating points of the ram air inlet 101 as a function of door position (measured as a percentage of full open).
As the graph 128 illustrates, the 2-piece inlet door 110 causes unstable flow in the diffuser passage 132 when the door is less than about 35% open. This instability can result in large pressure fluctuations, which in turn can impose excessive dynamic loads on the inlet components. The typical approach for dealing with this problem has been to strengthen the inlet components. The downside of this approach, however, is that it adds further complexity, cost and weight to the ram air inlets.